1. Field of the Invention
The present invention relates to a full color image forming method for use in electrophotographic image forming apparatus such as copiers, printers and facsimiles, and a color toner and intermediate transfer material useful for the method. More particularly, the present invention relates to an electrophotographic full color image forming method using an intermediate transfer material which receives a full color toner image from one or more image bearing members and transfers the full color toner image to a receiving material to form the full color toner image thereon. In addition, the present invention relates to a method for manufacturing the color toner.
2. Discussion of the Background
Image forming methods and apparatus are well known in which a plurality of color images formed on one or more image bearing members such as photoreceptors are transferred onto an endless image transfer material one by one (i.e., a first transfer process) and all the first transferred color toner images are second transferred onto a receiving material at a time (i.e., a second transfer process). In particular, such an image forming method using an intermediate transfer material is used for a full color image forming apparatus in which an original image, which is separated into a plurality of color images, is reproduced by overlaying color toner images such as a black, cyan, magenta, and yellow toner image.
In such an image forming method and apparatus, a problem which occurs is that image omissions, which look as if an image is eaten by worms, are observed in resultant toner images formed on a receiving material. This is because omissions are formed on toner images formed in the first and second transfer process. In order to avoid such a problem, i.e., in order to improve transferability of toner images, the following techniques have been proposed:
(1) Techniques Concerning Surface Roughness of Intermediate Transfer Material
In attempting to avoid image omissions, Japanese Laid-Open Patent Publication No. 3-242667 discloses a technique which uses an intermediate transfer material which is made of an elastomer and which has a specific surface roughness to improve adhesion of the intermediate material with a receiving material.
In addition, Japanese Laid-Open Patent Publications Nos. 63-194272, 4-303869, 4-303872 and 5-193020 have disclosed techniques which use an intermediate transfer material which has a specific surface roughness to improve adhesion of the intermediate material with a receiving material.
When toner images are transferred in the first and second transfer process, a transfer bias voltage is typically applied. Therefore, discharging tends to occur between an image bearing member and an intermediate transfer material and between the intermediate transfer material and a receiving material. If the intermediate transfer material has sharp projections on the surface thereof, the electric field applied to toner images on the projected portions of the intermediate transfer material are greater than that applied to toner images on the recessed portions of the intermediate transfer material.
This reason will be explained in detail referring to FIG. 1. Numeral 1 denotes an electrode having a smooth surface. Numeral 2 denotes an electrode having a serrated surface. The electrode 1 and electrode 2 face each other with a gap (Gp) therebetween. Numeral 3 denotes a projected portion of the electrode 2, and numeral 4 denotes a recessed portion of the electrode 2. When a transfer bias voltage is applied to the electrodes 1 and 2, discharging mainly occurs at gaps Gp1, Gp1xe2x80x2 and Gp1xe2x80x3 formed between the electrode 1 and the projected portions 3 because the electric field at the gaps Gp1, Gp1xe2x80x2 and Gp1xe2x80x3 is relatively high compared to any other gaps formed between the electrodes 1 and 2 including gaps Gp2 and Gp2xe2x80x2. This is because the distance at the gaps Gp1, Gp1xe2x80x2 and Gp1xe2x80x3 is the shortest. This is true for the case in which the electrode 2 is replaced with an intermediate transfer material and the electrode 1 is replaced with a receiving material or an image bearing member.
When toner images on the intermediate transfer material having a rough surface are transferred, toner particles positioned on the projected portions are present in a relatively large electric field compared to toner particles positioned on the recessed portions. In addition, the toner particles on the recessed portions have a relatively large adhesion compared to the toner particles positioned on the projected portions. Therefore, the toner particles on the projected portions are transferred relatively easily compared to the toner particles on the recessed portions.
FIGS. 2A to 2D are schematic views illustrating several different cases in which a toner particle (T) adheres to an intermediate material having a serrated surface. In FIG. 2A, a toner particle T contacts the flat surface of the intermediate transfer material. In FIG. 2B, a toner particle T contacts a projected portion of the intermediate transfer material. In FIGS. 2C and 2D, a toner particle T contacts a recessed portion of the intermediate transfer material. As can be understood from FIGS. 2A to 2d, the toner particles as shown in FIGS. 2C and 2D have a relatively large contact area with the intermediate transfer material compared to the toner particles as shown in FIGS. 2A and 2B. In these cases, if the projected portions and the recessed portions are made of the same material, there is relatively large van der Waals force between the toner particles and the recessed portions (i.e., in the cases as shown in FIGS. 2C and 2D) compared to the cases as shown in FIGS. 2A and 2B. Therefore, the adhesion between the toner particle and the recessed portion is greater than that between the toner particle and the plat or projected portion. The toner on the recessed portions tends not to be easily transferred, resulting in occurrence of omissions in the resultant toner images.
Therefore, it is preferable that the intermediate transfer material has a relatively smooth surface such that the intermediate transfer material does not cause omissions in the transferred toner images. This is also true for the image bearing member Namely, it is preferable that the image bearing member such as a photoreceptor has a relatively smooth surface such that the photoreceptor does not cause omissions in the transferred toner images. It is well known to prepare photoreceptor drums including Se as a photosensitive material having a relatively smooth surface to impart good toner transferability to the photoreceptor drum.
However, it is hard to prepare an intermediate transfer material having such a smooth surface. Therefore, it is hard to avoid the omission problem.
(2) Techniques Concerning Difference in Feeding Speed Between Intermediate Transfer Material and Image Bearing Member and Between Intermediate Transfer Material and Receiving Material
Japanese Laid-Open Patent Publication No. 2-213882 discloses a technique such that the feeding speeds of an intermediate transfer material and an image bearing member are specified to improve toner transferability and to avoid image omissions.
This technique will be explained referring to the first image transfer process (i.e., a toner image transfer process from an image bearing member to an intermediate transfer material).
When the intermediate transfer material has the same feeding speed as the image bearing member, it is needed to apply an electric force (i.e., a transfer electric field) to the toner image such that the toner image on the image bearing member, which adheres on the image bearing member due to the adhesion therebetween, is transferred to the intermediate transfer material only by the transfer electric field.
On the contrary, when the intermediate transfer material has a feeding speed different from that of the image bearing member, a mechanical force caused by the intermediate transfer material and the image bearing member which are fed at different speeds also acts to the toner image as well as the transfer electric field. Therefore, both the mechanical force and the transfer electric field can be applied to the toner image to transfer the toner image. Accordingly, this case is superior in toner transferability to the case in which the feeding speeds of the intermediate transfer material and the image bearing member are the same. Therefore, it is preferable for improving the image omission problem to use the method in which the intermediate transfer material has a feeding speed different from that of the image bearing member.
However, when the intermediate transfer material is fed at a feeding speed different from that of the image bearing member, the toner image tends to be distorted because the toner image is transferred while a shearing stress is applied thereto.
(3) Technique to Reduce Transfer Pressure
Japanese Laid-Open Patent Publications Nos. 1-177063 and 4-284479 have disclosed a technique in which the transfer nip pressure is specified.
This technique will be explained referring to the first image transfer process (i.e., a toner image transfer process from an image bearing member to an intermediate material).
When the first transfer process is performed, the image bearing member and the intermediate transfer material are pressed to each other by a mechanical force or an electrostatic force (this pressure is referred to as a transfer nip pressure). Namely, the toner that is present at a position between the image bearing member and the intermediate transfer material is pressed. When the toner is pressed, the toner particles are brought close to each other, resulting in increase of van der Waals force therebetween. In addition, the toner particles tend to aggregate, and therefore attraction between the materials constituting the toner particles also increases. Therefore, the toner tends not to be easily transferred.
From these reasons mentioned above, it is preferable that the transfer nip pressure should be decreased to improve the omission problem.
However, it is preferable to bring the intermediate transfer material close to the image bearing member of the receiving material for transferring the toner image to a desired position. From this viewpoint, the distance therebetween is preferably short. Therefore, there is a limit in decreasing the transfer nip pressure.
(3) Technique to Reduce Surface Energy of Intermediate Transfer Material
Japanese Laid-Open Patent Publications Nos. 2-198476 and 2-212867 have disclosed a technique in which the intermediate transfer material has a surface having a relatively small wettability to improve toner transferability thereof and to avoid the image omission problem.
At this point, the wettability means the adhesion between a liquid and a solid. The adhesion can be measured as an energy needed for separating the liquid from the solid. When the surface tension of the liquid is xcex3 A and the contact angle formed by the liquid and the surface of the solid is xcex8, the adhesion W between the liquid and the solid is represented by the following equation (1):
W=xcex3A(1+cos xcex8)xe2x80x83xe2x80x83(1)
The surface tension (critical surface tension) of a material X can be obtained as follows. Namely, a liquid having a surface tension of xcex3A1 is dropped on the material X to measure the contact angle (cos xcex81) formed by the liquid and the material X. This operation is performed with respect to various liquids having a different surface tension (xcex3An) to measure the contact angle (cos xcex8n). Points having coordinates of (cos xcex8n, xcex3An) are plotted on a graph (Zisman plot), and then the points are connected. This line is extended to determine the point at which the line crosses the line cos xcex8=1. The thus determined contact angle of the point, xcex3e, is the critical surface tension (i.e., the surface tension) of the material X.
In this case, when the wettability of various materials is measured using a liquid (for example, water), the surface tension xcex3A in equation (1) is constant because the same liquid (water) is used. Therefore, the wettability is proportional to the contact angle (cos xcex8). Namely, to measure the wettability of various materials is measured using the same liquid is to obtain the contact angle (cos xcex8) at the same surface tension. On the other hand, the line obtained by Zisman plotting is typically a straight line. In addition, the gradients of the straight lines obtained by Zisman plotting with respect to various material are similar. Accordingly, by measuring the contact angles of materials using a liquid, the wettabilities of the materials can be compared to each other.
Japanese Laid-Open Patent Publications Nos. 2-198476 and 2-212867 have disclosed a technique in which an intermediate transfer material having a small wettability, i.e., an intermediate transfer material having a small surface energy, is used in attempting to avoid the image omission problem.
In addition, Japanese Laid-Open Patent Publications Nos. 5-204255, 5-204257 and 5-303293 have disclosed a technique, in which an intermediate transfer material having multiple layers whose surface layer includes a material having good releasability is used in attempting to attempt to improve the toner transferability and to avoid the image omission problem. Further, Japanese Laid-Open Patent Publication No. 2-213881 discloses a technique in which a material having good releasability is applied to the surface of an intermediate transfer material. These techniques attempt to improve toner transferability of the intermediate transfer material by imparting good toner releasing ability to the intermediate transfer material. Adhesion between two different materials can be represented as a function of the surface tension thereof. It is well known that the greater the surface tension, the greater the adhesion therebetween (e.g., between the toner and the intermediate transfer material). At this point, the surface tension is the synonym as the surface energy when pure materials are used. Therefore, the surface tension of a material, which is not a pure material, is used as a proxy of the surface energy as well as the proxy of the wettability.
In the technique mentioned above in paragraph (4), each of the adhesions between the toner and the image bearing member, between the toner and the intermediate transfer material, and between the toner and the receiving material is a combination force in which all physical forces such as electrostatic force, van der Waals force etc. are totaled.
By using this technique, the toner transferability can be improved in the second image transfer process. However, the toner transferability cannot be necessarily improved in the first image transfer process.
(5) Technique to Improve Toner Transferability by Refreshing Surface of Intermediate Transfer Material
Japanese Laid-Open Patent Publications Nos. 5-273893, 5-307344, 5-313526 and 5-323802 have disclosed a technique in which it is attempted to maintain good toner transferability of an intermediate transfer material, i.e., it is attempted to avoid the image omission problem, by refreshing the surface of the intermediate transfer material to avoid toner filming. When the surface tension of an intermediate transfer material is reduced ideally, toner filming does not occur and therefore this technique (5) is not needed. Namely, the technique (5) is a supplementary technique for the technique (4).
In the second transfer process, the image omission problem tends to occur when roller transferring is performed, i.e., a roller is used as a transfer device. The reason is considered as follows:
(a) When a full color image is transferred, a large mechanical adhesion, which is a non-Coulomb force between the toner and the intermediate transfer material, is generated due to the pressure applied by the roller. The large adhesion is also caused by a thick toner layer because a full color toner image has a relatively large thickness. In detailed description, when the toner is pressed by a roller while an intermediate transfer material is present therebetween, the effective density of the toner is increased. In addition, since the toner particles are brought close to each other, the van der Waals force therebetween increases. Therefore, the adhesion of the toner to the intermediate transfer material increases.
(b) In a process in which an image forming process is repeatedly performed, when a toner filming problem occurs, i.e., when a toner film is formed on the surface of an intermediate transfer material, adhesion force is generated between the intermediate transfer material and the toner. In general, the intermediate transfer material is made of a material having a small surface tension or a small surface energy to avoid the toner filming problem. Even when such a material is used for the intermediate transfer material, adhesion (i) corresponding to the surface tension between the intermediate transfer material and the toner is generated. If toner filming once occurs, the adhesion between the intermediate transfer material and the toner changes to the adhesion force (ii) between the toner particles. At this point, it is obvious that the adhesion force (ii) is greater than the adhesion force (i). Therefore, toner images tend to adhere to the filmed toner, resulting in occurrence of the image omission.
In attempting to solve the image omission problem in the second transfer process, U.S. Pat. No. 5,053,827 (Method and apparatus for intermittent conditioning of a transfer belt) discloses the following method.
The method includes a conditioning process in which a roller (conditioning mean), which is constituted of a fluorine-containing material which has a surface energy smaller than that of an intermediate transfer belt, is brought into contact with the intermediate transfer belt to reduce the surface energy of the intermediate transfer belt.
In addition, the U.S. Patent discloses an embodiment including an intermediate transfer belt constituted of polycarbonate. In the embodiment, the initial surface energy of the intermediate transfer belt is from 37 to 38 dyne-cm. When the conditioning process is not performed, the surface energy of the intermediate transfer belt increases to 40 to 45 dyne-cm. When the surface energy is greater than 40 dyne-cm, an image transfer problem occurs. Therefore, as mentioned above, a roller constituted of a fluorine-containing material having a surface energy not greater than 30 dye-cm is brought into contact with the intermediate transfer belt to form a thin layer of the fluorine-containing material on the surface of the intermediate transfer belt, resulting in decrease of the surface energy of the intermediate transfer belt. In the US patent, it is described that when the surface energy is too low, an image transfer problem tends to occur in the first image transfer process, i.e., when a toner image is transferred from an image bearing member to the intermediate transfer material.
When the present inventor performs an experiment in which an intermediate transfer belt constituted of polycarbonate is used for the intermediate transfer material (19) of an image forming apparatus as shown in FIG. 3, the image omission problem occurs in the second image transfer process when the intermediate transfer belt is used for a long time. When a proper amount of a lubricant (zinc stearate) is applied to the intermediate transfer belt, the image omission problem in the second transfer process can be improved. However, the resultant copy image has low image density, i.e., the apparatus produces unclear copy images (hereinafter referred to as an unclear image problem) because the quantity of a toner image per unit area is small. When the present inventor examines the reason, it is found that an image transfer problem occurs in the primary transfer process.
When the present inventor performs an experiment in which an intermediate transfer belt constituted of ETFE (ethylene-tetrafluoroethylene copolymer) is used, the unclear image problem occurs from the start of the copying test. The reason for the unclear image problem is considered to be as follows. Although the surface energy of the intermediate transfer belt is controlled at a specific range by performing the conditioning process, the surface energy of the image bearing member gradually increases as the copying test is continuously performed even when the surface of the image bearing member is cleaned with a cleaning brush or roller. This is because (1) a toner adheres to the surface of the image bearing member; and (2) the surface of the image bearing member is contaminated by gases such as ozone, NOx and the like gases. When the surface energy of the image bearing member increases, the toner image formed on the image bearing member tends to remain thereon when the toner image is transferred to the intermediate transfer material, resulting in occurrence of the unclear image problem.
In addition, when the surface energy of the image bearing member decreases, another type transfer problem which tends to occur is that a color toner image transferred from the image bearing member to the intermediate transfer material is re-transferred to the image bearing member when another color toner image is transferred from the image bearing member or another image bearing member to the intermediate transfer material, resulting in occurrence of image omissions.
Thus, when the surface energy of the image bearing member is relatively high compared to that of the intermediate transfer material, toner images to be transferred from the image bearing member to the intermediate transfer material tend to remain on the image bearing member without being transferred, or toner images once transferred on the intermediate transfer material tend to be re-transferred to the image bearing member. The reason for that the image omission problem occurs in the initial copy images when an intermediate transfer material constituted of ETFE is used as mentioned above is considered to be that the surface energy of the image bearing member is much greater than that of the intermediate transfer material.
In attempting to avoid such a problem, the U.S. patent discloses a technique in which the conditioning process is performed only when the surface energy of the image bearing member becomes too high. Specifically, the U.S. patent proposes a method that the conditioning process is performed after predetermined copies are reproduced.
However, it is inconvenient that the conditioning process is performed while a copying operation is suspended. In addition, the surface energy of the image bearing member is not necessarily in the predetermined range when predetermined copies are reproduced. Further, a device for coating a lubricant on the intermediate transfer material and a device for polishing the intermediate transfer material are needed for the image forming apparatus, and thereby the apparatus becomes complex and the costs of the apparatus increase.
Several methods for improving the image omission problem by a toner are known. For example, a method is proposed in which toner transferability is increased by enhancing the fluidity of the toner, and a method in which a toner including resin particles is used to prevent the toner particles from being adhered to each other when the toner is transferred upon application of pressure thereto. However, when the fluidity of a toner is excessively increased, the toner images tend to scatter when the toner images are transferred, resulting in deterioration of image reproducibility.
Because of these reasons, a need exists for an image forming method which can produce color images having good image qualities such as good image reproducibility and which does not cause the image omission problem.
Accordingly, an object of the present invention is to provide a full color image forming method which can produce color images having good image qualities such as good image reproducibility and which does not cause the image omission problem.
Another object of the present invention is to provide a color toner for use in the color image forming method mentioned above, and a method for manufacturing the toner.
Yet another object of the present invention is to provide an intermediate transfer material for use in the color image forming method mentioned above.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a full color image forming method including the steps of forming an electrostatic latent image on an image bearing member; developing the latent image with a color developer including a color toner to form a color toner image thereon; first transferring the color toner image onto an endless intermediate transfer material while applying a developing bias voltage; repeating the latent image forming step, latent image developing step and color toner transfer step several times using a plurality of color developers to prepare a full color image on the intermediate transfer material; and second transferring the full color toner image to a receiving material, wherein the amount of each color toner image formed on the image bearing member is from about 0.4 mg/cm2 to about 1.5 mg/cm2, and the absolute value of the charge quantity of each color developer is not less than 15 xcexcC/g, and wherein the following equation is satisfied with respect to each of the first color toner transferring steps:
5.4xc3x97Q1+90 less than Vbxe2x88x92V1 less than 5.4xc3x97Q1+150
wherein Vb represents the developing bias voltage (V), V1 represents a potential (V) of a background area of the electrostatic latent image formed on the image bearing member, on which the color toner image is not to be formed, and Q1 represents the absolute value of the charge quantity of each color developer. At this point, (Vbxe2x88x92V1) is referred to as a developing potential.
The color toner image is preferably formed on the image bearing member by developing the electrostatic latent image using a reverse developing method and a developing unit including a plurality of developing devices which rotates and each of which includes a magnetic brush and one of the color developers.
As another aspect of the present invention, a color toner is provided for the image forming method, which has a charge rising property Z not less than 70%, wherein the charge rising property Z is represented as follows:
Z=(Q20/Q600)xc3x97100
wherein Q20 represents a charge quantity of the toner when a carrier including the toner in an amount of not greater than 5% by weight is agitated for 20 seconds at a temperature of from 15 to 25xc2x0 C. and a relative humidity of from 25 to 80%, and Q600 represents a charge quantity of the toner when the carrier including the toner in an amount of not greater than 5% by weight are agitated for 600 seconds at the condition.
The toner preferably includes a fluidity imparting agent including a particulate hydrophobic silica and a particulate hydrophobic titanium oxide, each of which has a particle diameter not greater than 0.05 xcexcm.
The toner preferably has a circular degree of from 0.93 to 0.97 when the circular degree is measured with respect to the toner particles which remain on a sieve of 500 mesh having an opening of 26 xcexcm after sieving the toner with the sieve. In addition, the toner particles remaining on the sieve of 500 mesh after sieving the toner of 100 g is preferably not greater than 10 mg.
As yet another aspect of the present invention, a method for manufacturing the color toner is provided which includes the steps of first pulverizing a kneaded toner mixture using a jet pulverizer including a collision plate and capable of blowing compressed air to prepare a particulate toner mixture; and second pulverizing and classifying the particulate toner mixture using a rotor pulverizer and a classifier which is connected with the rotor pulverizer while the pulverized toner mixture circulates through the rotor pulverizer and the classifier, wherein the rotor pulverizer includes a container and a rotor, which are concentric.
As a further aspect of the present invention, an intermediate transfer material for use in the image forming method is provided which has a volume resistivity of from 109 to 1013 xcexa9xc2x7cm and whose surface has a friction coefficient not greater than 0.4.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.